This invention relates to ultrasonic diagnosis and imaging of the body with ultrasonic contrast agents and, in particular, to new methods and apparatus for ultrasonically detecting and imaging with contrast agents.
Ultrasonic diagnostic imaging systems are capable of imaging and measuring the physiology within the body in a completely noninvasive manner. Ultrasonic waves are transmitted into the body from the surface of the skin and are reflected from tissue and cells within the body. The reflected echoes are received by an ultrasonic transducer and processed to produce an image or measurement of blood flow. Diagnosis is thereby possible with no intervention into the body of the patient.
Materials known as ultrasonic contrast agents can be introduced into the body to enhance ultrasonic diagnosis. Contrast agents are substances which strongly interact with ultrasonic waves, returning echoes which may be clearly distinguished from those returned by blood and tissue. One class of substances which has been found to be especially useful as an ultrasonic contrast agent is gases, in the form of tiny bubbles called microbubbles. Microbubbles present a significant acoustic impedance mismatch in the body, and nonlinear behavior in certain acoustic fields which is readily detectable through special ultrasonic processing. Gases that have been stabilized in solutions in the form of tiny microbubbles are infused into the body and survive passage through the pulmonary system and circulate throughout the vascular system. Microbubble contrast agents are useful for imaging the body""s vascular system, for instance, as the contrast agent can be injected into the bloodstream and will pass through the veins and arteries of the body with the blood supply until filtered from the blood stream in the lungs, kidneys and liver.
One type of microbubble contrast agent currently under investigation comprises coated microbubbles. The microbubbles of the contrast agent are covered with a thin biodegradable coating or shell. The microbubbles have diameters between 0.1 xcexcm and 4.0 xcexcm and a specific density about {fraction (1/10)} of the density of water. The coated microbubbles are suspended in an aqueous solution for infusion into the blood stream.
Coated microbubbles have the advantage of being stable in the body for a significant period of time, as the shells serve to protect the gases of the microbubble from diffusion into the bloodstream. The size of the microbubbles is chosen to enable the microbubbles to pass through capillary beds in the body.
At moderately high sound pressure amplitudes the acoustic pressure waves can cause the shells of coated microbubbles to rupture, freeing the bubbles to behave as noncoated microbubbles until they diffuse into the bloodstream. In their noncoated form acoustic energy can induce nonlinear motion of the microbubbles, itself a detectable ultrasonic phenomenon. This acoustically induced destruction and collapse of the microbubbles produces a high amplitude response and a characteristically bright pattern in the color Doppler mode. Hence color Doppler is an advantageous modality for detecting the collapse of contrast agent microbubbles.
U.S. Pat. No. 5,456,257, assigned to the same assignee as the present invention, describes a technique for detecting microbubbles through phase insensitive detection of microbubble destruction and differentiation of the detected signals on a spatial basis. Phase insensitive contrast agent detection advantageously reduces artifacts from moving tissue, and also performs well when imaging contrast agent perfused tissue, where the contrast agent is finely distributed and moving slowly through the fine capillary structure of tissue. It is desirable to be able to perform contrast agent imaging with equal effect in large, rapidly moving blood pools such as the chambers of the heart. It is also desirable to specifically tailor the operation of the ultrasound machine to harmonic characteristics when performing harmonic contrast imaging.
In accordance with the principles of present invention, new and improved apparatus and methods for the detection and imaging of ultrasonic contrast agents are provided. Ultrasonic apparatus is provided for coherent imaging of ultrasonic contrast agents, which is advantageous in blood pool contrast imaging. In a second embodiment, the apparatus is specially tailored to be programmed with response characteristics suitable for harmonic contrast agents. The inventive apparatus also includes a display for simultaneously viewing a real time image which displays anatomical structures for localization of the contrast agent and a triggered contrast image displaying contrast enhanced images. Methods of employing the inventive apparatus with contrast agents include the measurement of perfusion rate characteristics, mtiltizone contrast imaging, a technique for discerning larger vessels in a bed of fine capillary structures, multifrequency contrast imaging, the display of contrast enhanced tissue, and a technique for the elimination of artifacts occurring during high PRF contrast image acquisition.